I've got a 91 that has been pretty reliable since I bought it in 95. The only problem I had was with the fuel system. The Jeep would suddenly stop running, after much troubleshooting and frustration it turns out that the underhood relay that controls the fuel system was failing. The relay box is located right next to the engine so heat was probably a factor. Once I replaced the $6000 dollar relay no more problems. Most of my other problems have been self inflicted.

I have a 94 YJ, and the only 2 problems that I have experienced with it is that that Fuel filter had rusted out so that costs about 40$ and the Parking Brake kept slipping. That was a recall that happened not too long ago so you can get that fixed for free. So thats all ive experienced.

I have a 94YJ 4.0 with 115,000+ miles on it. No major problems with it. Just minor stuff like rear pinion seal leaking (going to a Ford 8.8 w/disc) Engine compression test are well within the limits. Clutch(no sure if it is original, doubt it) no problems. Fires right up, not sluggish and has enough power for what I like to do.
I did have to replace both front hub assemblies. Quick and easy.
I change the oil and filters regularly. Spark plugs get changed also. My engine rarely sees over 3000rpm unless on the trail and every once in a while on the street.

To sum it up, my 94 has been real reliable and trustworthy. LIke JeepX stated the other stuff was self inflicted

If you get one, look at the underside and see if there are any dents on the skid pan. And of course have a mech check it out.